This invention relates to a method of applying a powder coating to a length of a lignocellulosic material, such as for example a sheet of paper.
Powder coating is the term given to the application of a decorative coating principally to metallic articles. The coating is applied to the article in an electrostatic field by propelling dry pigmented particles from a special gun, which is friction or electrostatically activated, towards the article, the particles being attracted to the article by electrostatic forces. The particles adhere to the surface of the article, and depending upon the force of the electrostatic field, successive particles adhere until the required build up is achieved, whereafter any surplus powder falls from the article and may be recovered. The article is then moved through a suitable oven at elevated temperatures, usually In the range of 140.degree. C., to 185.degree. C., or at lower temperatures in the presence of ultra violet light, to cause the powder particles to melt, flow, coalesce and cure lo form a coating.
The advantages of powder coating are that a wide variety of textures and surface finishes may be achieved, the coatings are very tough and resistant to wear and in exterior grades, resistant to weathering. In addition, the powder coating method is solventless and because the powder can be recovered for reuse, wastage is virtually nil. The thickness of the coating on the article may be very accurately controlled. Further, the method is of particular application to articles of complex shape. Powder coatings are further characterised by their flexibility and adhesion so that, after powder coating, an article such as a flat sheet may be post formed over curves or edges.
One powder coating technique requires that the article to be coated must be able to sustain an electrostatic field for the particles of the powder coating composition to adhere thereto. It is possible that an article which does not retain an electrostatic field could be dampened or wetted in order for the particles of the powder coating composition to adhere to the article. However, oven heating of the article may lead to the commencement of decomposition, or to "blowing" as gasses escape trough the coalescing powder film from the heated articles. The alternative is fusion coating wherein the article is preheated before applying the powder coating, such as in a fluidized bed.
There is thus a need for a method by which articles which normally cannot be powder coated, can have a powder coating applied thereto,